Semiconductor coherent laser beam sources have found many industrial and commercial applications in recent years. For example, lasers are used in telecommunications, in optically readable media pickups that are used in CD players, CD ROM drives and DVD players and in medical imaging. In particular wide area coherent lasers would be very useful in holographic displays, in communication systems and in information processing. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, so it is necessary to cleave a wafer into chips and package the chip before knowing if the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter “VCSELs”) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs are comprised of a gain medium between two periodic stacks of binary-layered medium, giving a periodic profile of the refractive index variation. VCSELs combine the performance advantages of edge-emitting lasers at costs comparable to LED solutions. VCSELs emit light vertically from the wafer surface, like LEDs, which means their fabrication and testing is fully compatible with standard I.C.s procedures and equipment, and also means that arrays of VCSELs are feasible. Additionally, VCSELs are much faster, more efficient, and produce a smaller divergence beam than LEDs.
The VCSELs structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) 2-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, the surface-emitting VCSELs has a radially symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSELs technology allows the fabrication of two-dimensional laser arrays.
However, VCSELs suffer from a number of disadvantages. The manufacture of VCSELs requires sophisticated and expensive microfabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors. These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric constant difference between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by the absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas and in producing large area arrays. The materials typically used for VCSELs do not have the desired low absorption and high index contrast over a broad frequency range. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns.
In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. The density of photon modes is not changed appreciably by use of low index contrast multilayer Bragg reflector and the gain cannot be improved in a VCSELs system as compared to that in an ordinary laser cavity. Also, an external device must be used to control the polarization of the light.
With respect to wider area coherent lasers, since the maximum excitation energy is proportional to the laser area, large-area thin-film devices provide a new approach for high-power lasers. While it would appear that VCSELs are the best candidate for wide area lasing in a 1-D periodic structure, high order transverse modes arise in small-diameter VCSELs, while in large-diameter VCSELs spontaneous filamentation results from structural nonuniformities. Furthermore, in all previously known lasers coherence width is much smaller than longitudinal size (VCSELs) or mirror distances (in conventional lasers). It should also be noted that VCSELs inherently do not allow for large area coherence because their length is greater than the diameter, and because they are comprised of alternating layers with low index contrast. This requires a greater number of layers and hence a thicker structure.
It would thus be desirable to provide a laser apparatus and method that produces a wide-area coherent laser beam superior to other previously known wide area coherent laser beam sources. It would further be desirable to provide a wide area coherent lasing apparatus and method that is configurable for using in filtering and amplification applications.